A constant composition gray gas and a constant composition nongray gas radiation model are developed and applied it? computational fluid dynamic simulations of all industrial scale steam cracking furnace. Both models are based oil the exponential wide band model. The gray gas model simplification, commonly used for simulations of industrial applications, is found to have all effect on predicted variable fields like flue gas flow, temperature, and heat flux to the reactor tubes. When the nongray gas model is used, higher energy absorption by the flue gas in the furnace and lower energy transfer to the process gas in the reactor tubes is calculated because of the high absorption coefficients in the strongly absorbing bands of 2.7 and 4.3 mu m. Thus, the calculated thermal efficiency increases from 37.5% when using the nongray gas model to 42.6% when using the gray gas model. A 5% difference in the thermal efficiency is large considering the scale and the importance of the process and should be taken into account by the furnace designer. It is also shown that although both models reproduce the basic characteristics of the flow pattern in the furnace, quantitative differences in the flue gas speed are predicted in some regions of the furnace domain. (c) 2007 American Institute of Chemical Engineers.