TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Method: After synthesis gas, the water content in the synthesis gas is separated and recovered as condensed water which is recycled to the arc-plasma reactor. The synthesis gas is pressurized before supplying to the methanation-reaction tank.    TECHNOLOGY FOCUS - MECHANICAL ENGINEERING - Preferred Components: The plasma arc reactor further has 3-phase-alternate-current electrodes supported in the insulated casing, and a ground electrode from the current electrodes. Several arc plasma are generated simultaneously between the current electrodes and the ground electrodes .The manufacturing apparatus further has a cooling system, steam separator for separating water content as condensed water from methane, and recycle line for circulating the condensed water. EXAMPLE - A solid-form carbon material was filled in the plasma reactor and micro arc route was formed in gap between carbon materials. A poly-phase alternating-current electric power was applied to the arc electrodes of the plasma reactor, and a micro arc plasma was generated in the micro arc route. Input voltage was controlled, so that the temperature in the reactor was maintained at 800-1000 degreesC. The generation ratio of hydrogen was set to 2-5 with respect to carbon monoxide. Water or water vapor was supplied to the micro arc route. Water vapor and carbon of the carbon material were reacted in presence of arc plasma, and a synthesis gas containing hydrogen and carbon monoxide was generated. The obtained synthesis gas was extracted and cooled. The synthesis gas was pressurized and introduced to a methanation-reaction tank containing the methanation catalyst ,and methane gas was obtained. Methane gas obtained was cooled and pressurized, and condensed water was separated from the methane gas. Methane gas was converted into acetylene in the microwave plasma reactor. The obtained acetylene and methane were mixed in a ratio of 0.05-5 mol with respect to 100 mols of acetylene, and were reacted in presence of super strong acid catalyst pellet and n-butane was obtained. The obtained n-butane was contacted with a zeolite catalyst and was converted into propane by polymerization.