This paper presents an evaluation of the weighted-sum-of-gray-gases (WSGG) model in the computation of the radiative heat transfer in an axisymmetric gas system composed of H2O, CO2 and soot by comparison with line-by-line (LBL) integration. The test cases consider temperature and species concentrations fields that are representative of a laminar diffusion jet flame of ethylene diluted with H2O. Different approaches are considered in the application of the WSGG model, including the use of WSGG coefficients obtained for different ratios between the mole concentrations of H2O and CO2, and the superposition between the coefficients of H2O/CO2 and soot. The WSGG coefficients for H2O and CO2 are based on HITEMP2010 database, while for soot they are based on the consideration of linear spectral dependence of its absorption coefficient. The spatial integration of the radiative transfer equation is carried out with the discrete ordinates method. The results in the paper show that, although the ratio between the mole concentrations of H2O and CO2 varies locally in the flame, using WSGG coefficients for a constant ratio, but equivalent to the global average ratio in the flame, can provide satisfactory solutions in comparison to the LBL integration. Moreover, the superposition method between the WSGG coefficients of combined H2O/CO2 and of soot proved accurate considering both moderate and high concentrations of soot. The paper provides algebraic relations for the temperature and concentration fields, which can be used for the evaluation of other gas models in future studies. (C) 2018 Elsevier Ltd. All rights reserved.