A model of the steam gasification of a single char particle driven by high-intensity radiation was developed and experimentally verified with available measurements in the literature. This was used to explore the sensitivity of the particle surface temperature and heat-transfer mechanisms to variations in particle diameters, radiative heat flux, and the concentration of the gasification agent H2O under typical conditions for solar gasification reactors. The results highlight the importance of the particle diameter in influencing solar-to-chemical energy conversion efficiency and assist in the selection of appropriate feedstock particles to match the conditions in specific solar gasification reactors.