The effect of temperature profiles of gases, concentration profile of species, surface reflectivity and specularity of surfaces on the wall heat flux is investigated for a parallel plate geometry enclosure. The Mayer-Goody band model in conjunction with a modified Curtis-Godson approximation to account for the inhomogeneous path, and the just overlapping line model, are employed for integration of the radiation transport equation. Three absorbing-emitting species, H₂O, CO₂ and carbon particles (soot), mixed with air are considered. Non-gray calculations are performed with 25 cm^-1 spectral resolution using mostly experimental data. Some typical results and detailed conclusions are presented.