Partial oxidation and non-catalytic steam reforming of methane is studied by conducting an experimental and numerical analysis of reactions taking place in the post-flame region of a generic laminar atmospheric burner operated at fuel-rich conditions. Laminar planar premixed methane/oxygen/nitrogen (CH4/O2/N2) flames are stabilized above a porous material with an equivalence ratio Pdbl = 1.8. The oxygen enrichment Ω in the O2/N2 mixture varies from 0.32 to 0.45. Mixtures are also diluted with superheated steam. The water vapor mole fraction XH2O u in the unburnt mixtures ranges from 0.11 to 0.28. The temperature of burnt gases is measured along the central axis of the burner with a thermocouple. Gas chromatography measurements along the same axis indicate the production of hydrogen (H2) and carbon monoxide (CO) just downstream the flame front. Measurements also reveal a decrease of the CO concentration as the distance to the flame front increases due to a drop of the temperature in the burnt gases. One-dimensional direct simulations of flames using detailed chemistry mechanisms are conducted under non-adiabatic conditions accounting for conductive heat losses to the burner surface and radiative heat transfer of semi-transparent gases using a statistical narrow-band model for the radiative properties of the main species (H2O, CO2, CO,...). Predicted temperature profiles and major species mole fractions yield satisfactory match with measurements only if radiative heat losses are taken into account in the simulations. It is shown that self-absorption of thermal radiation from H2O and to a lesser extend from CO2 fully controls the temperature profile in the post-flame region. The endothermic steam methane reforming reaction taking place in this region is strongly penalized by radiative heat loss. It is shown that increasing the steam dilution promotes the H2/CO ratio within the burnt gases, while rising the oxygen enrichment benefits to the CO production to the detriment of H2. This analysis reveals the important role of radiative heat transfer and self-absorption of steam and to a lesser extent carbon dioxide in the syngas formation such as for example the methane auto-thermal reforming process.