Film cooling is a promising technique for maintaining the combustor liner under safe temperatures. The radiative heat transfer increases as the fourth power of temperature and cannot be neglected at high combustion temperatures. In the present study, a numerical investigation is conducted on the effect of combustion gas radiation, mainstream combustion gases obtained by lean, stoichiometric, and rich combustion, emissivity (ε_L) of the liner surface, and thermal barrier coating on the film cooling performance. A practical three-dimensional slot geometry used for film cooling of an annular combustor is considered in the present study. High-temperature-resistant yttria-stabilized zirconia (YSZ) is used to simulate a typical thermal barrier coating (TBC) of the actual engine. The numerical simulation results indicate that the radiation adversely affects film cooling effectiveness. For blowing ratio (BR) of 0.5, density ratio (DR) of 2.6, and liner surface emissivity (ε_L) of 0.8, there is a 23.5% reduction in the area-averaged effectiveness compared to without considering radiation. In addition, the effect of combustion gases obtained for different equivalence ratios (∅) shows a minimal effect on film cooling effectiveness. Radiation is dependent on the emissivity, and the film cooling performance is enhanced with a decrease in the liner surface emissivity. The application of thermal barrier coating enhances the effectiveness by 12% compared to the case without TBC.