Abstract: Fuel gas mixtures of different composition containing hydrogen may be the main products or by-products of chemical conversion processes such as, without limitation, processes for the conversion of solid or liquid fuels. Such gas mixtures are, however, normally not fit for public gas supplies, as their heating value and their density and other properties would normally not conform to the standards and the codes of practice for gas properties. Methanation is a known process for the conversion of hydrogen-rich fuel gas mixtures into gases conforming to standard specifications. During the exothermal reaction of methanation, more than 15% of the enthalphy of the reactants may be lost.The invention allows the increase of the heating value of hydrogen-rich fuel gas mixtures, said increase being achieved at a high efficiency and maximizing the recovery of useful energy in the form of gas conforming to standards and electric power.