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 10C 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.