NOVELTY - A chamber (34) of a thermal plasma reactor (34) is supplied with solid carbon materials to form minute arc passages (35). Steam passed through the passages reacts with the materials in the presence of arc discharge plasmas created by supplying electric power to arc electrodes (36,38,40) in the chamber, to produce synthesis gas which is introduced into a methanation catalyst to produce a substitute natural gas (SNG).    USE - For producing a substitute natural gas using synthesis gas.    ADVANTAGE - The process method provides high-quality SNG from solid carbon materials and feed water, economically. Feed water and solid carbon materials which are extremely low in cost are utilized as raw materials, resulting in remarkable reduction in production cost of SNG. The utilization of thermal plasma reactor which is compact but has a high operating performance enables sufficient production of SG in a large volume, thereby increasing the production efficiency of SNG. Since the solid carbon materials are consumed only for producing SG and not as reformer fuels, the utilization rate of the solid carbon materials is extremely high. Since the H2/CO ratio is easily adjustable, the SNG production system is controlled in operation to provide an optimum operating control. The thermal plasma reactor is precisely controlled at an appropriate temperature by merely varying the voltage of power supply, providing a quick response enabling mass production of SNG at maximum efficiency. Condensed water is recycled as feed water to the reactor, thereby eliminating environmental pollution. SG is produced instantaneously, allowing the SNG production system to be started up and terminated in operation in quick response which is advantageous for an emergency stop such as earthquake. The SNG production system is compact but high in operating efficiency, therefore sizable investment is eliminated.    DETAILED DESCRIPTION - A thermal plasma reactor (14) having a chamber (34) provided with arc electrodes is prepared, and the chamber is supplied with solid carbon materials to form a large number of minute arc passages (35) in the carbon materials. Electric power is supplied to the arc electrodes to create arc discharge plasmas in the minute arc passages, respectively. Steam is passed through the passages to react with the solid carbon materials under the presence of the arc discharge plasmas to produce synthesis gas (SG) containing hydrogen (H2) and carbon monoxide (CO). SG is introduced into a methanation catalyst of a methanation reactor to synthesize substitute natural gas (SNG).    An INDEPENDENT CLAIM is included for an SNG production apparatus, comprising:    (1) an arc plasma reactor having a solid carbon supply port, a feed water supply port (52), an insulating casing formed with a synthesis gas outlet (86), an arc plasma chamber formed in the insulating casing, arc discharge electrodes (36,38,40) located in the arc plasma chamber, and several minute arc passages formed in the solid carbon materials filled in the arc plasma chamber;    (2) a feed water supply pump for supplying feed water into the feed water supply port to convert the feed water into steam;    (3) an arc discharge power supply (16) for supplying electric power to the electrodes to generate arc discharge plasmas in the passages such that steam reacts with the solid carbon materials to produce SG; and    (4) a methanation reactor having a methanation catalyst for converting the SG into SNG.    DESCRIPTION OF DRAWING(S) - The figure is a cross-sectional view of an arc plasma reactor forming portion of the substitute gas production system.    Thermal plasma reactor (14)    Arc discharge power supply (16)    Chamber (34)    Steam generating zone (34A)    Minute arc passages (35)    Arc electrodes (36,38,40)    Feed water supply port (52)    Synthesis gas outlet (86)