Radiation heat transfer within a solar chemical reactor for the co-production of zinc and syngas is analyzed by the Monte Carlo ray-tracing method. The reactor is treated as a 3D nonisothermal cavity-receiver lined with ZnO particles that are directly exposed to concentrated solar irradiation and undergo endothermic reduction by CH4 at above 1300 K. The analysis includes coupling to conduction/convection heat transfer and chemical kinetics. A calculation of the apparent absorptivity indicates the cavity's approach to a blackbody absorber for either diffuse or specular reflecting inner walls. Numerically calculated temperature distributions, Zinc production rates, and thermal efficiencies are validated with experimental measurements in a solar furnace with a 5-kW prototype reactor. At 1600 K, the zinc production rate reached 0.12 mol/min and the reactor's thermal efficiency exceeded 16%. Scaling up the reactor to power levels of up to 1 MW while keeping constant the relative geometrical dimensions and the solar power flux at 2000 suns results in thermal efficiencies of up to 54%.