A method was proposed to couple the heat source with ORCs (Organic Rankine Cycles). The integrated-average temperature difference, ΔTave, quantifies the thermal match in the evaporator. ΔTave decreases with increase in critical temperatures (Tc). The fluid with Tc approaching Tgas,in (flue gas inlet temperature) can have ΔTave → 0.5 × (Tgas,out − T5), which is called the optimal temperature difference, at which the thermal efficiency is maximum. The ORC performance with fluids of Tc > Tgas,in has less deviation from the optimal condition. Thermal efficiencies are well correlated with critical temperatures. The ORC thermal efficiencies are randomly distributed against other fluid physical properties except Tc. Thus, the critical temperature can be the sole criterion for the fluid selection, as far as the thermal efficiency is concerned. The organic fluids with Tc in the range of (Tgas,in − 20–30 K, Tgas,in + 100 K) are recommended. Specific fluids are recommended for heat source temperature in the range of (100–300 °C) by screening 57 fluids. More fluids are available for ORCs with low heat source temperatures. Limited fluids are available for high temperature heat source applications. Due to the expanded fluids in region II, some fluids such as R245fa can be used over a wide heat source temperature range. •The integrated-average temperature difference quantifies the thermal match for the evaporator.•An optimal temperature difference was identified at which the thermal efficiency is maximum.•Critical temperature can be the sole criterion for selection of working fluids for subcritical ORCs.•Working fluids were recommended for flue gas driving ORCs in the range of 100–300 °C.