Computational Fluid Dynamics (CFD) simulations are performed to model the radiative heat transfer in natural gas fired furnaces operating under oxygen enriched conditions with flue gas recirculation. Gray and non-gray formulations of recently proposed weighted-sum-of-gray-gas (WSGG) models are employed to compute the gas absorption coefficients within two representative furnaces (OXYFLAM and HTAC) and their accuracies and impact on the predicted velocity, temperature and radiative heat fluxes evaluated by comparisons against experimental data. As a result of the nearly homogeneous and isothermal conditions encountered within the furnaces, the differences in the predicted incident radiative flux profiles between the gray and non-gray models is not significant. However, in the OXYFLAM furnace where higher temperatures and larger temperature gradients are encountered, the differences between the gray and non-gray radiation calculations manifest themselves in the temperature distributions downstream of the flame region.