After 30 years of waterflooding, distributed temperature sensors (DTS) data on an oilfield has recorded up to 50% reduction in reservoir temperature from its initial value of 120°C. Literature review showed previous efforts are more inclined towards interpretating DTS data to determine layer contribution in a single well rather than studying the impact of temperature change on the field performance and recovery factor. Changes in reservoir temperature will cause a change in reservoir fluids properties and fluids mobilities and subsequent recovery factor will be impacted. In order to determine the impact of reservoir cooling on IWAG (immiscible water-alternate gas) performance, a comprehensive study has been performed combining coreflood experiment with a 3D reservoir modeling. 3D thermal reservoir model has been constructed and calibrated for the purpose of determining the temperature distribution across the field. The model indicates that with higher injection volume of water; reservoir temperature is lowered with temperature distribution highly influenced by the reservoir properties, injected volume, and fluid properties. The coreflood experiments have been performed under isothermal and reduced temperature using a stack of native cores with average properties closely match the reservoir rock that carry most of the oil in place. The results showed an incremental IWAG recovery factor of up to 4% (STOIIP) higher in the reduced temperature experiment compared to IWAG experiment under isothermal conditions. This research paper has demonstrated that ignoring the effect of reservoir cooling in IWAG project can lead to an underestimated IWAG recovery factor and eventually impacting the overall project economics.