A method of liquefaction/conditioning of a compressed gas/condensate flow extracted from a petroleum deposit, for transport in liquefied form with a transport vessel, especially for such processing of a compressed gas/condensate flow which has been separated from a crude oil extracted from an offshore oil field. The gas/condensate flow is depressurized and cooled in several steps for producing a stabilized liquefied natural gas (LNG) and a stabilized liquefied petroleum gas (LPG), for transport thereof in separate tanks. Disclosed is also a gas expansion plant for execution of the method, and a system for handling and processing of a natural gas from an offshore petroleum field, comprising a production ship to which there is supplied a well stream from an underground source, a field plant installed on the production ship, for processing of the well stream received on the production ship, a vessel for transport of liquefied gas fractions, a high-pressure pipeline arranged for transfer of compressed gas from the field plant to the vessel, and a gas expansion plant according to the invention installed on the transport vessel.