New techniques of radiative-convective heat transfer calculation are proposed which allow one to solve problems for any optical thickness of a radiating volume. The application of these techniques significantly reduces the required computer time as compared to that demanded by other techniques. Comparison of the results obtained with those by exact calculation has shown that for the case of radiating layer being optically thick the radiant thermal conductivity approximation allows one not only to describe correctly the structure of the flow but also to determine radiation flux with an error not exceeding 10 per cent. In this case heat transfer by radiation leads to formation of temperature boundary layer near volume boundary.