The present invention relates to a process plant and method for cooling and optionally liquefaction of a product gas, particularly for liquefaction of natural gas, based on a closed loop of multi-component refrigerant in heat exchange with the gas to be cooled and optionally condensed. The process plant is comprises at least one primary heat exchanger (20) arranged to cool the product gas directed to the heat exchanger (10), at least one compressor (46) arranged to compress the low level refrigerant directed from the first of the at least two secondary heat exchangers (64), at least one pre-cooling heat exchanger (54) to sub-cool and partly liquefy the compressed refrigerant, at least one phase-separator (60) arranged to separate the partly liquefied multi-component refrigerant into a more volatile fraction and a less volatile fraction, at least two secondary heat exchangers (64, 114), the first of the at least two secondary heat exchangers (64) arranged to cool the more volatile fraction from the phase-separator (62), and the second of the at least two secondary heat exchangers (114) arranged to cool further the more volatile fraction, a throttling device (118) arranged to reduce the pressure of a part of the more volatile fraction to become the low level refrigerant to be heat exchanged in the second of at least two secondary heat exchangers, a throttling device (76) arranged to reduce the pressure of a part of the more volatile fraction to become the low level refrigerant to be heat exchanged in the at least one primary heat exchanger (20), a throttling device (102) arranged to reducing the pressure of the less volatile fraction from the at least one phase-separator (60) to become part of the low level refrigerant, for mixing with the low level refrigerant from the at least one primary heat exchanger (20), and the low level refrigerant from the second of at least two secondary heat exchangers (114) this directed to heat exchange through the first of at the least two secondary heat exchangers (64).