Abstract: A combined system for producing fuel and thermal energy, comprising: an electrolyser (150) for producing oxygen and hydrogen in the process of water electrolysis; a gasifier (110) for producing synthesis gas in a process of gasification of carbon-based fuel in the presence of a gasifying agent; a methane synthesis reactor (130) for producing methane in a process of synthesis of carbon oxide from the gasifier (110) and hydrogen from a water electrolyser (150); a reactor (120) with a catalytic packing for producing carbon dioxide in a process of combustion of synthesis gas from the gasifier (110) and/or methane; wherein the electrolyser (150) comprises a heat exchange system connected with a heat exchanger (160).