A natural gas liquefaction system comprising: at least a pre-cooling loop, through which a first refrigerant is adapted to circulate, the pre-cooling loop comprising: at least one compressor for pressurizing the first refrigerant; at least one prime mover for driving said compressor; at least one condenser for removing heat from the first refrigerant; at least a first expansion element (119A-119D) for expanding the first refrigerant; at least a first heat exchanger (123A-123D) for transferring heat from natural gas to the first refrigerant; and at least a cooling loop, downstream of said pre-cooling loop, where through a second refrigerant circulates, the natural gas being adapted to be sequentially cooled in the pre-cooling loop and in the cooling loop; wherein said first refrigerant comprises a gas with a molecular weight greater than 35; wherein the pre-cooling loop further comprises: a plurality of sequentially arranged first expansion elements (119A-119D) configured for expanding the first refrigerant at a plurality of decreasing pressure levels; a plurality of first heat exchangers (123A-123D) arranged and configured for receiving respective portions of said first refrigerant expanded through said plurality of sequentially arranged first expansion elements (119A-119D) and for transferring heat from the natural gas to the first refrigerant; and a plurality of return paths (130A-130D) returning side streams of the first refrigerant from each said first heat exchangers (123A-123D) to a respective compressor stage (109A-109D) of the compressor; and characterized in that said compressor is an integrally geared turbo-compressor comprising a plurality of compressor stages (109A-109D) each one being provided with an independent set of movable inlet guide vanes (11 OA-110D) for autonomously regulating flows entering in the compressor stages; and in that said compressor further comprises at least one intercooler (153, 155,157) between at least two serially arranged compressor stages arranged between the delivery side of one of the plurality of compressor stages (109|A-109D) and the suction side of another of the plurality of compressor stages (109A-109D). at least one compressor for pressurizing the first refrigerant; at least one prime mover for driving said compressor; at least one condenser for removing heat from the first refrigerant; at least a first expansion element (119A-119D) for expanding the first refrigerant; at least a first heat exchanger (123A-123D) for transferring heat from natural gas to the first refrigerant; and at least a cooling loop, downstream of said pre-cooling loop, where through a second refrigerant circulates, the natural gas being adapted to be sequentially cooled in the pre-cooling loop and in the cooling loop; wherein said first refrigerant comprises a gas with a molecular weight greater than 35; wherein the pre-cooling loop further comprises: a plurality of sequentially arranged first expansion elements (119A-119D) configured for expanding the first refrigerant at a plurality of decreasing pressure levels; a plurality of first heat exchangers (123A-123D) arranged and configured for receiving respective portions of said first refrigerant expanded through said plurality of sequentially arranged first expansion elements (119A-119D) and for transferring heat from the natural gas to the first refrigerant; and a plurality of return paths (130A-130D) returning side streams of the first refrigerant from each said first heat exchangers (123A-123D) to a respective compressor stage (109A-109D) of the compressor; and characterized in that said compressor is an integrally geared turbo-compressor comprising a plurality of compressor stages (109A-109D) each one being provided with an independent set of movable inlet guide vanes (11 OA-110D) for autonomously regulating flows entering in the compressor stages; and in that said compressor further comprises at least one intercooler (153, 155,157) between at least two serially arranged compressor stages arranged between the delivery side of one of the plurality of compressor stages (109|A-109D) and the suction side of another of the plurality of compressor stages (109A-109D). a plurality of sequentially arranged first expansion elements (119A-119D) configured for expanding the first refrigerant at a plurality of decreasing pressure levels; a plurality of first heat exchangers (123A-123D) arranged and configured for receiving respective portions of said first refrigerant expanded through said plurality of sequentially arranged first expansion elements (119A-119D) and for transferring heat from the natural gas to the first refrigerant; and a plurality of return paths (130A-130D) returning side streams of the first refrigerant from each said first heat exchangers (123A-123D) to a respective compressor stage (109A-109D) of the compressor; and characterized in that said compressor is an integrally geared turbo-compressor comprising a plurality of compressor stages (109A-109D) each one being provided with an independent set of movable inlet guide vanes (11 OA-110D) for autonomously regulating flows entering in the compressor stages; and in that said compressor further comprises at least one intercooler (153, 155,157) between at least two serially arranged compressor stages arranged between the delivery side of one of the plurality of compressor stages (109|A-109D) and the suction side of another of the plurality of compressor stages (109A-109D).