The effect of real gas volumetric radiation on the thermal development in laminar parallel plate channel flow of H2O and/or CO2 in the case of gas cooling has been investigated numerically. The nongray radiation effects of the gas have been treated using a global spectral approach, the Spectral Line Weighted-sum-of-gray-gases model. The results reveal that gas radiation results in significantly higher total heat transfer to the cooled channel wall, with an attendant more rapid drop in gas mean temperature. Gas radiation is seen to increase the local convective and total (radiative plus convective) Nusselt number for increasing radiating species mole fraction for both H2O and CO2 and for increasing gas inlet temperature. The influence of gas radiation on the thermal development is less pronounced for CO2 than for H2O. An apparent thermally fully developed condition may exist for this combined convection-radiation problem with real gases in the gas cooling scenario, and radiation has the effect of significantly extending the thermally developing region. Combined hydrodynamic and thermal development yields higher heat transfer than the thermally developing condition. Smaller channel wall spacing results in lower radiative heat transfer and the aforementioned radiation effects are diminished. Local convective and radiative flux and thermal entry length also increase with elevated gas total pressure.