NOVELTY - The assembly comprises: a methanation reactor (6) to convert carbon dioxide into methane using catalytic hydrogenation; an electrolyzer device (5) to produce the necessary hydrogen to be introduced into the methanation reactor; a unit (2) to collect soil gas comprising carbon dioxide from degassing zones; and a first treatment device (3) configured for desulfurizing the soil gas coming from the collection unit and for purifying the carbon dioxide to be introduced into the methanation reactor. The electrolyzer device is supplied with electric energy coming from renewable sources (4).    USE - The assembly is useful for producing methane (claimed).    ADVANTAGE - The assembly simply and economically produces methane with high degree of purity (99.5 vol.%).    DETAILED DESCRIPTION - The assembly comprises: a methanation reactor (6) to convert carbon dioxide into methane using catalytic hydrogenation; an electrolyzer device (5) to produce the necessary hydrogen to be introduced into the methanation reactor; a unit (2) to collect soil gas comprising carbon dioxide from degassing zones; and a first treatment device (3) configured for desulfurizing the soil gas coming from the collection unit and for purifying the carbon dioxide to be introduced into the methanation reactor. The electrolyzer device is supplied with electric energy coming from renewable sources (4). The assembly further comprises: a unit for generation of electric energy from renewable sources; a second treatment device to dehydrate methane; and a line to transport water produced by the methanation reactor to the electrolyzer device. The collection unit comprises: a layer of material permeable to the soil gas; a layer of material impermeable to the soil gas arranged above the permeable layer and to prevent outflow of the soil gas; and a suction system to guarantee both the progress of the soil gas through the permeable layer and draw-off of the soil gas. The first treatment device uses a technique of adsorption on sorbents at a temperature of greater than or equal to 250 degrees C. An INDEPENDENT CLAIM is included for a method for production of methane.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of an assembly for producing methane.    Collection unit (2)    First treatment device (3)    Renewable sources (4)    Electrolyzer device (5)    Methanation reactor. (6)