I used a heat gun to solder a 32 pin SOIC IC (DIR9001) and after I was done I saw that a cap nearby (solid cap) got EXTREMELY hot and started to discolor (yellowish hue).
Is it possible that the cap died? It hasn't exploded or something.
The temperature to which you set your temperature controlled heat-gun is about 100 degrees C higher than the recommended reflow profile for ceramic capacitors.
A normal vapor/air reflow profile looks more like this, where Peak Temperature is between 230-260C:
There is more information available in these documents:
Basically, you want to be careful to avoid thermal shock cracks, which occur when the part is heated too quickly. Move the heat gun in from a distance over the course of a minute rather than directly exposing it to 360C air.
Also, avoid heating the capacitor to much over 260 C for any length of time. This temperature is the general maximum for most parts, including silicon parts like the SOIC you replaced.
What exactly do you mean by "heat gun"? If this is not temperature controlled, like a hot air soldering station, then you probably damaged a lot of things, not just the capacitor. Obviously this thing made air hot enough to melt solder, but you don't know how much hotter. If it is not temperature controlled, then using it to solder was a really bad idea.
The fact that a nearby part got hot enough to discolor means you really overdid it. Everything in the vicinity of the IC you abused should be considered suspect and replaced, including the IC. Even if a part appears to work, it could be damaged in subtle ways and cause hard to diagnose problems.
You ruined a bunch of parts and possibly the board under them. Replace them and move on.
I used a temperature controlled heat-gun of course! It is analog and not digital though, and I set it to about 300-360C.
I ended up replacing the capacitor (never checked if it works, though) and the board works fine.
Soldering with a non-temperature-controlled heat gun (and whatever other non-controlled tool) is generally a bad practice, because there is no way to ensure that the proper temperatures are used. It depends on the power, the distance, the conductivity of the material compared to air...
So it's quite likely that you exceeded the maximum temperature for that component, and it's recommended to consider it broken. Probably the same helds for the IC, which is not better suited for this kind of operations.
If you want to do reflow soldering, consider using a good oven and the proper solder paste. And check ALL the datasheet for reflowing temperature profiles.
So it was a temperature controlled heat gun (good), but you also exceeded the normal reflowing temperature (bad) of around 50 to 100 degrees (see Kevin's graph). I'd suggest you to check also the FR4 temperature specifications, because you may have damaged also the PCB itself. And of course replace (if keeping the board) all the components that have been heated to that temperature.
Of course it is possible that the capacitor damaged. To deny that statement is to assert that it is impossible for the capacitor to be damaged, which is absurd, given that it was hot enough for its finish to discolor.
The capacitor should be assumed to be toast and replaced. It's a waste of time trying to confirm whether it is good or bad since it takes less time to just put in a new one and lay all doubt to rest.
All electronic components have threshold operating temperatures. It is possible that you damaged the capacitor from prolonged exposure. You could have also heated some flux residue which could cause the discoloration. If you are worried just find a relevant datasheet and check the operating temperatures. If it looks like you could have damaged it you can easily find a replacement.