NOVELTY - The process comprises partly pre-cooling natural gas by heat exchange with a pre-cooling refrigerant contained in a pre-cooling cycle (123), liquefying natural gas by heat exchange with a liquefying refrigerant contained in a liquefying cycle (100), and sub-cooling the natural gas by heat exchange with a sub-cooling refrigerant contained in a sub-cooling cycle. The liquefying refrigerant comprises nitrogen and an additional compound of which a boiling point at atmospheric pressure is -20 degrees C to -60 degrees C and a crystallization temperature is less than or equal to -80 degrees C.    USE - The process is useful for producing liquefied natural gas from sea (claimed).    ADVANTAGE - The process is capable of energy-efficiently and simply producing the liquefied natural gas from sea with improved performance.    DETAILED DESCRIPTION - The process comprises partly pre-cooling natural gas by heat exchange with a pre-cooling refrigerant contained in a pre-cooling cycle (123), liquefying natural gas by heat exchange with a liquefying refrigerant contained in a liquefying cycle (100), and sub-cooling the natural gas by heat exchange with a sub-cooling refrigerant contained in a sub-cooling cycle. The liquefying refrigerant comprises nitrogen and an additional compound of which a boiling point at atmospheric pressure is -20 degrees C to -60 degrees C and a crystallization temperature is less than or equal to -80 degrees C. A molar proportion of nitrogen in the liquefying refrigerant is less than or equal to 98.5%. The liquefying refrigerant is expanded in expanding turbines, and is partially condensed in the expanding turbines. The pre-cooling cycle, liquefying cycle and/or sub-cooling cycle comprises a compressor driven by gas turbine generating fumes. A part of heat from fume is transferred to an absorption refrigeration machine, and the gas turbine produces fume at a temperature of less than or equal to 510 degrees C and is aero-derivative gas turbine. The pre-cooling refrigerant is partly cooled by the absorption refrigeration machine. The turbine gas is fed into air and the air is cooled before feeding. INDEPENDENT CLAIMS are included for:    (1) an installation for producing liquefied natural gas; and    (2) an use of a composition comprising carbon dioxide and hydrofluorocarbon for cooling and liquefying natural gas.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of an installation for producing liquefied natural gas.    Liquefying cycle (100)    Natural gas supply conduit (101)    Pre-cooling exchanger (104)    Liquefying exchanger (105)    Pre-cooling cycle. (123)