first thing the LM358 op amp is not suited for this application because of the thermopile sensors can give negative voltages, these opamp cannot pickup this.so its better to go with a better rail to rail opamp.I am trying to make a breakout board for a thermopile sensor for the past 2 months.i had tried lm358,AD620 opamp and everthing but it did not give a readable value.So its better to go with an opamp like LMC6482 which is a cmos rail to rail opamp.