A computational platform was developed to study radiation of high-temperature gases during atmospheric reentries. This platform includes a 2-D axisymmetrical Navier–Stokes solver and a radiative heat transfer solver based on a Monte Carlo method. In this paper, only gas radiation occurring during entries in the Martian atmosphere is considered. Gas radiative properties are obtained using a statistical narrowband model for CO and CO2 molecules in high-temperature and low-pressure conditions. Computations on a test case related to the Mars sample return orbiter enabled us to set up a calculation methodology that includes a loose coupling of aerothermodynamics and radiative transfer. Monte Carlo band model calculations are validated against line-by-line calculation and a ray-tracing method. Results show that radiative fluxes on the rear part are important in comparison with convective fluxes. They are mostly due to CO2 infrared radiation. The effects of radiative transfer on the flow structure and temperature fields in both the front shield and the wake regions are discussed.