We have developed a high efficiency process for producing a substitute natural gas using LPG and naphtha as raw materials. In the process, sulfur poisoning of the steam reforming catalyst is protected and the amount of catalyst used can be reduced applying a high performance desulfurization technology developed. This technology enables the removal of trace amounts of sulfur which cannot be removed by a conventional hydro-desulfurization method. In addition, we have developed a high performance steam reforming catalyst which has a resistance to carbon deposition even under low S/C conditions. The catalyst makes it possible to reduce excessive steam for preventing carbon deposition, and to operate with high thermal efficiency.