NOVELTY - Separation of air in a system involves compressing air in a compressor (1) to send first part of the air to a combustor (5) and its second part to an air separation device (20) forming an oxygen enriched gas (21) and nitrogen enriched gas (16). At least part of nitrogen enriched gas is compressed and send upstream to an expander (17) coupled to a cycle compressor (22) of a refrigeration cycle of a natural gas liquefaction unit.    USE - In air separation particularly integrated with a gas turbine process and a natural gas liquefaction process.    ADVANTAGE - The work produced by the expander operates the cycle compressor of the refrigeration cycle. The expander is also coupled with a gas turbine, natural gas conversion unit and air separation unit. Steam derived from the natural gas conversion unit is expanded in a turbine to generate energy that is used to drive at least one compressor (e.g. main air compressor, nitrogen compressor, oxygen compressor, booster) or their respective motors present in air separation unit. The process reduces the costs of a production complex, which simultaneously produces from the same natural gas source, both liquefied natural gas and a product of the conversion of the natural gas (e.g. methanol, dimethyl ethers or a Fischer Tropsch product), by integrating air separation unit, gas turbine, natural gas conversion unit and natural gas liquefaction unit.    DETAILED DESCRIPTION - At least part of oxygen enriched gas is sent to a natural gas conversion unit (23). The natural gas source (25) sends a first stream of natural gas to the natural gas conversion unit and second stream to the natural gas liquefaction unit.    An INDEPENDENT CLAIM is included for an integrated apparatus for the process.    DESCRIPTION OF DRAWING(S) - The figure shows the system used in air-separation process.    compressor (1)    combustor (5)    nitrogen enriched gas (16)    expander (17)    air separation unit (20)    oxygen enriched gas (22) cycle compressor (21)    natural gas conversion unit (23)    natural gas source (25)    liquefier. (28)