Thermopile temperature sensor
Here is the datasheet.
The NTC sensor resistance between pins 2 and 4 should vary with the actual temperature of the device, about -5% per degree C (as shown on the chart), and 100K at 25°C. This is to be used to compensate the millivolts out of pins 1 and 3.
The latter (which reacts to the actual temperature being measured vs the sensor) is about +40uV or 50uV/°C so you would want a better op-amp than the LM358 for sure, in terms of Vos and TCVos. Also watch the bias current, since the
source resistance is quite high (as much as 150K at 25°C). The offset of the LM358 can be as much as +/-9mV which corresponds to something like +/-150°C measured temperature.
In general you will calculate the device temperature from the NTC sensor resistance and then calculate from the curve in figure 1 the millivoltage corresponding to that temperature. You read the millivoltage off the thermopile, then add the millivoltage from the NTC calculation, then calculate the resulting temperature using the 25°C graph in figure 1.
Edit:
Ignoring nonlinearity (which the slightly complex procedure above deals with properly), the thermopile measures (approximately) the temperature difference between the remote temperature being measured and the sensor ambient, and the NTC sensor measures the sensor ambient. So if the thermopile indicates a millivoltage that corresponds to 50°C (with the assumption of a 25°C sensor temperature) and the sensor temperature is actually 35°C, then the remote temperature is approximately 60°C.