A numerical study of combined forced convection and gas radiation from a circular cylinder is carried out. The fluid is assumed to be radiatively participating. Simulations are carried out for both laminar and turbulent flows. The Reynolds numbers considered are 100 and 500 for laminar flow and 7190, 21,580, 35,950, and 50,350 for turbulent flow. To check the effect of three-dimensionality on heat transfer characteristics, numerical simulations are also carried out for three-dimensional geometry. All the thermophysical properties of the working fluid are taken as those of air and are assumed to be constant for the range of temperature considered in the present study. In order to understand the effects of absorption and scattering of thermal radiation, a detailed parametric study is carried out by varying the absorption and scattering coefficients from 0 to 60 (m(-1)). It is observed that the contribution of radiative heat transfer on total heat transfer is significant when the flow is laminar. The radiative Nusselt number decreases with the increase of absorption coefficient.