Radiative heat transfer in, (a) two model enclosures containing a high-temperature CO2-H2O misture, and (b) a real multiburner natural-gas-fired furnace, was studied using the simple grey gas model and the more realistic weighted-sum-of-grey-gases model of gas radiative properties. The radiative transfer equation was solved by the discrete-ordinates method. When the radiative transfer equation and the enthalpy transport equation are solved simultaneously, the simple grey gas model yields wall heat flux distributions in reasonably good agreement with those of the weighted-sum-of-greygases model but underpredicts gas temperature levels. When the temperature field is specified and the radiative transfer equation is then solved, however, the wall heat flux distributions predicted using the simple grey gas model are in serious error compared to those from the weighted-sum-of-grey-gases model. The weighted-sum-of-grey-gases model predicts more accurate heat flux and gas temperature distributions than the simple grey gas model in the full modelling of the gas-fired furnace.