Systems and methods for producing product gas fuel and solid char fuel from a carbon-containing feedstock are described. Feedstock is introduced into and transported through at least one substantially horizontal sublimation reaction chamber. An initial and final sublimation temperature is maintained within less than ±10° C. in an atmosphere free from external oxygen and externally supplied catalyst. The system is configured to not have any product gas leak out of the reaction chamber or oxygen leak into a hot box configured to heat the reaction chamber.