NOVELTY - The production of a mixture containing hydrogen, methane, carbon dioxide and water comprises reacting carbon-containing substrates with supercritical water in a reactor (8) at a temperature of 400-1000degreesC and pressure of 221-400 bar in auto-catalytic operation or in presence of catalysts. The carbon contained in the substrate is oxidized partially by oxygen, which originates from the decomposition of the supercritical water or is dissolved in the initial mixture. The reaction is carried out in a diffusion-welded plate-type heat exchanger.    USE - For production of hydrogen, methane, carbon dioxide and water.    ADVANTAGE - The procedure ensures simple, pure and effective production of hydrogen, methane, carbon dioxide and water, and avoids colorific losses as well as ensures high heat transmission for heating the reaction mixture.    DETAILED DESCRIPTION - The production of a mixture containing hydrogen, methane, carbon dioxide and water, comprises reacting carbon-containing substrates with supercritical water in a reactor (8) at 400-1000degreesC and at 221-400 bar in auto-catalytic operation or in presence of catalysts. The carbon contained in the substrate is oxidized partially by oxygen, which originates from the decomposition of the supercritical water or is dissolved in the initial mixture. The reaction is carried out in a diffusion-welded plate-type heat exchanger. The heating of the reactant mixture and the cooling of the formed product mixture take place under heat recovery through heat exchange in a heat exchanger, in which a hydraulic diameter of the flow channels is 0.5-15 mm. The heat exchanger is predominantly operated in a counter-current mode and has average driving temperature gradient of 0.2 to 50degreesC. The product discharged from the reactor is cooled down at 20-60degreesC and the gaseous products are separated at 35-70 bar. The carbon dioxide contained in the gaseous product is separated through distillation or absorption. The obtained carbon dioxide is supplied to a further material recycling or a sequestration step. The reactant mixture is superheated at the end of the heating stretch for reaching the endothermic heat of reaction. A part of the developing reaction gases are burned for the purpose of heating and the hot incineration gases are led over the heat exchanger, in which the reactant mixture is heated. The product gases such as hydrogen, methane and carbon dioxide are separated in a distillation column operated under positive pressure into a hydrogen/methane-rich fraction over the head, carbon dioxide-rich fraction over a lateral discharge and a water fraction as sump product. The distillate separation takes place in a partition column or in an arrangement of thermally-coupled distillation columns. The product gases are released from the cooled down reaction discharge by decompression at a lower pressure of 30-50 bar, and separated in a washing column that is exposed to water or an aqueous mixture, into a hydrogen and methane-containing head product as well as essentially a carbon dioxide and water-containing sump product, which by heating up and/or decompression can be further separated into nearly pure carbon dioxide and water fractions. The formed product gases are used as raw material for the production of hydrogen, after a partial separation of the contained carbon dioxide. Coal, liquid fuels, gaseous fuels, waste materials or their mixtures are used as carbon-containing substrates. The salt layers separating themselves from the heat exchanger surfaces are detached by cavitation forces. The flow channels exhibit periodic cross-section contractions and cross section expansions and water is led through the flow channels in cleaning processes. The ratio of the smallest to the largest flow cross-section is 0.5-0.8. Suspended salt particles, metal chips or ceramic(s) particles are added to the reactant mixture to prevent the building of salt deposits or to remove already formed salt deposits. An INDEPENDENT CLAIM is included for a device for the production of a mixture containing hydrogen, methane, carbon dioxide and water.    DESCRIPTION OF DRAWING(S) - The diagram shows procedure for the production of a mixture:    Crusher; (2)    Roughing filter; (4)    High-pressure pump; (6)    Reactor; (8)    Separating device. (11)