A method for deep desulfurization of synthesis gas comprising introducing carbonaceous material and optionally steam into a gasifier comprising a heat transfer media, extracting a first heat transfer stream comprising heat transfer media and optionally unconverted carbonaceous material from the gasifier, and introducing at least a portion of the first heat transfer stream into a combustor, introducing oxidant and optionally a fuel into the combustor, extracting a second heat transfer stream comprising heat transfer media from the combustor, and introducing at least a portion of the second heat transfer stream into the gasifier, introducing a compound capable of reacting with sulfur to produce sulfate, sulfide or both, extracting a purge stream comprising ash, sulfate, halide, or a combination thereof from the second heat transfer stream, extracting a flue gas from the combustor, and extracting a gasifier product synthesis gas stream comprising less than 1000 ppm sulfur from the gasifier.