NOVELTY - The method involves purifying gas, and liquefying the gas by cooling the gas at a temperature lower than a liquefaction temperature of the gas in a heat exchanging unit (10) that is arranged adjacent to a production point and mounted on a vehicle. Negative kilocalories coming from a medium (5) are utilized by the heat exchanging unit for cooling the gas in the heat exchanging unit, where the medium is stored in a container adjacent to the heat exchanging unit. The liquefied gas is transported towards a use point. The medium is prepared in a medium preparation unit (13).    USE - Method for transporting gas such as methane-containing gas (claimed) i.e. methane-containing liquefied natural gas, from a production point towards a use point through a methane tanker in a port.    ADVANTAGE - The heat exchanging unit is arranged adjacent to the production point, and the negative kilocalories coming from the medium are utilized by the heat exchanging unit for cooling the gas in the heat exchanging unit, where the medium is stored in the container adjacent to the heat exchanging unit, thus enabling liquefaction of the gas in a compact manner. The medium is prepared without energy expenditure, thus enabling reduction of environmental impact. The method enables insulation of the container, thus avoiding dispersion of the negative kilocalories.    DETAILED DESCRIPTION - The vehicle is designed as a river transport vehicle e.g. barge and boat, a road transport vehicle i.e. lorry (6), a railway vehicle i.e. railway goods wagon, or an air vehicle. The medium is air or nitrogen in a liquid form and/or carbon dioxide in liquid or solid form or a mixture of two forms. An INDEPENDENT CLAIM is also included for a gas liquefaction installation.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of a gas liquefaction installation. '(Drawing includes non-English language text)'    Liquefaction installation (1)    Medium (5)    Lorry (6)    Heat exchanging unit (10)    Medium preparation unit (13)