This work applies a computational fluid dynamics (CFD) approach to examine gas and soot-related radiation mechanisms in air and oxy-fuel flames operated with propane as fuel. In oxy-fuel combustion, CO2 and H2O replace the N-2 in air combustion. As a result, the radiative heat transfer characteristics differ between the combustion atmospheres. Moreover, changes in soot formation have been observed in oxy-fuel compared to air-fired flames. Both gas- and soot-related radiation can be essential for the design of oxy-fuel furnaces and need to be accounted for when temperature and heat transfer conditions are modeled. The aim of the work is to determine the respective impact of combustion gases and soot in heat transfer modeling of the flames. Both gray and nongray approaches are used to account for the gas radiation and the results are compared to measured data from a 100 kW oxy-fuel unit to investigate if a gray model is sufficient to generate a reliable solution when applied in CFD simulations of oxy-fuel combustion. In addition, calculations of the radiative source term are performed for a domain between two infinite plates, with temperature and concentration profiles from the CFD simulations of the present work. It is shown that the nongray approach accurately predicts the source term in both combustion environments, whereas the gray model fails in predicting the source term. The source term has a direct influence on the temperature field in CFD calculations. However, this work also shows that the inclusion of soot radiation is more critical in sooty air and oxy-fuel flames than the use of a more rigorous description of the radiative properties of the gas.