A method for liquefying an industrial gas or a hydrocarbon gas, or any mixture of industrial or hydrocarbon gases, comprising the following non-sequential steps: (a) receiving a gas having an inlet pressure and compressing or decompressing the gas to a desired pressure; (b) chilling the gas through at least one absorption chiller; (c) adiabatically reducing the pressure of the gas to liquefy at least a portion of the gas; (d) heating a rich aqua-ammonia fluid in a rectifier to liberate ammonia gas, using one or a combination of recovered heat of compression energy if the gas is compressed in step (a), or trim heat which may be produced by direct or indirect combustion heat exchange, or other available waste heat recovery streams with the necessary temperature and mass flow conditions, producing a lean aqua-ammonia fluid; (e) subcooling the lean aqua-ammonia fluid and circulating it to the top of a vapour absorption tower; (f) condensing ammonia gas from the rectifier and flashing the liquid ammonia to produce chilled ammonia gas for use in the at least one absorption chiller; (g) absorbing ammonia gas from the at least one absorption chiller into the lean aqua-ammonia fluid in the VAT to produce the rich aqua-ammonia fluid for step (d), characterised in that the method further comprises performing step (g) while maintaining a sub atmospheric operating pressure at the top of the VAT, by pinching a lean aqua-ammonia flash valve thereby reducing the pump suction pressure of a rich aqua-ammonia pump but maintaining the suction pressure above the net positive suction head required (NPSHR) of the pump removing the rich aqua-ammonia fluid from the VAT, and maintaining a subcooled lean aqua-ammonia solution. (a) receiving a gas having an inlet pressure and compressing or decompressing the gas to a desired pressure; (b) chilling the gas through at least one absorption chiller; (c) adiabatically reducing the pressure of the gas to liquefy at least a portion of the gas; (d) heating a rich aqua-ammonia fluid in a rectifier to liberate ammonia gas, using one or a combination of recovered heat of compression energy if the gas is compressed in step (a), or trim heat which may be produced by direct or indirect combustion heat exchange, or other available waste heat recovery streams with the necessary temperature and mass flow conditions, producing a lean aqua-ammonia fluid; (e) subcooling the lean aqua-ammonia fluid and circulating it to the top of a vapour absorption tower; (f) condensing ammonia gas from the rectifier and flashing the liquid ammonia to produce chilled ammonia gas for use in the at least one absorption chiller; (g) absorbing ammonia gas from the at least one absorption chiller into the lean aqua-ammonia fluid in the VAT to produce the rich aqua-ammonia fluid for step (d), characterised in that the method further comprises performing step (g) while maintaining a sub atmospheric operating pressure at the top of the VAT, by pinching a lean aqua-ammonia flash valve thereby reducing the pump suction pressure of a rich aqua-ammonia pump but maintaining the suction pressure above the net positive suction head required (NPSHR) of the pump removing the rich aqua-ammonia fluid from the VAT, and maintaining a subcooled lean aqua-ammonia solution.