A Multi-Zone Radiation method was employed in a conceptual simulator of gas turbine combustion chamber, which is configured as tubular, reverse flow and no film cooling, and equiped with a lean, premixed, low swirl and low Nox burner. Such simplified simulator is typically used for conceptual trade-off and optimization studies during the pre-design phase, when there is no experimental data available and when the number of runs can reach the order of hundreds or more. In the standard procedure, the convection heat fluxes of the annulus passage and combustion chamber liner are approximated by semi-empirical correlations; the thermal radiation flux is estimated by use of viewing factors between adjacent zones only; the heat conduction is estimated by a one-dimensional model. However, the present paper introduces a more accurate and theoretically rigorous estimation of the radiative heat transfer by adopting a Multi-Zone Method. The numerical code inputs are: the geometry; the flame burnout profile at the combustion region; and the mass fluxes through the dilution holes and the burner. The main outputs of the simulator are the average temperature and heat fluxes by conduction, radiation and convection of each zone. The present modeling strategy enabled the authors to assess the basic design characteristics of a conceptual model of combustion chamber, such as its length, its inlet air temperature, its combustion region equivalence ratio, its flame burnout profile, and its wall emissivities. Finally, it has advantages over the simplified radiation models because it takes into account the effects of temperature, burn-out and composition variations of all zones on each zone and vice-versa.