To understand the effect of soot on heat transfer in a combustion furnace, a combined heat transfer analysis in a methane-air diffusion combustion furnace including soot was carried out. Nongray radiative properties of the combustion gas were evaluated using the Exponential Wide-Band Model, which was applied to the gray band model. The Discrete-Ordinates Method was used to solve the radiation transport equation. As the soot concentration increases, the radiative heat flux to the combustion furnace walls increases, whereas the convective heat flux decreases. The temperature in the furnace falls with an increase in the soot concentration, and both peaks of the temperature and the total wall heat flux shift to the upstream. This is because net irradiation of the radiation energy from the combustion gas-soot mixed medium increases with the soot concentration. Since the medium radioactivity decreases with the reduction in temperature accompanying the increase in the soot concentration, the wall surface thermal radiation flux decreases in the downstream of the furnace.