A method for maintaining at a predetermined flow rate value and at a predetermined temperature value the liquefied natural gas ("LNG") outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant ("MRL") cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant ("MRV") cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising the steps of: comparing said predetermined LNG flow rate value with the actual LNG flow rate; comparing said predetermined LNG temperature value with the actual LNG temperature; (i) comparing a predetermined value of the warm end temperature difference with the actual warm end temperature difference, wherein the warm end temperature difference is the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream (124, 134 or 100) entering said warm end selected from MRL, MRV and the natural gas feed, and/or (ii) comparing a predetermined value of the mid-point temperature with the actual mid-point temperature, wherein the mid-point temperature is the temperature of a stream at a location between the liquefying and subcooling sections of the heat exchanger means; varying (132 or 138), by an amount corresponding to the difference between the actual and predetermined LNG flow rates, one of the MRL and MRV flow rates; varying (138 or 132) the other of the MRV and MRL flow rates to maintain an MRL/MRV ratio, which ratio is determined by one of (a) the difference between the actual and predetermined LNG temperatures and (b) the difference between the actual and predetermined warm end temperature differences or mid-point temperatures; and varying, by an amount corresponding to the other of (b) the difference between the actual and predetermined warm end temperature differences or mid-point temperatures and (a) the difference between the actual and predetermined LNG temperatures, the actual LNG flow rate. comparing said predetermined LNG flow rate value with the actual LNG flow rate; comparing said predetermined LNG temperature value with the actual LNG temperature; (i) comparing a predetermined value of the warm end temperature difference with the actual warm end temperature difference, wherein the warm end temperature difference is the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream (124, 134 or 100) entering said warm end selected from MRL, MRV and the natural gas feed, and/or (ii) comparing a predetermined value of the mid-point temperature with the actual mid-point temperature, wherein the mid-point temperature is the temperature of a stream at a location between the liquefying and subcooling sections of the heat exchanger means; (i) comparing a predetermined value of the warm end temperature difference with the actual warm end temperature difference, wherein the warm end temperature difference is the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream (124, 134 or 100) entering said warm end selected from MRL, MRV and the natural gas feed, and/or (ii) comparing a predetermined value of the mid-point temperature with the actual mid-point temperature, wherein the mid-point temperature is the temperature of a stream at a location between the liquefying and subcooling sections of the heat exchanger means; varying (132 or 138), by an amount corresponding to the difference between the actual and predetermined LNG flow rates, one of the MRL and MRV flow rates; varying (138 or 132) the other of the MRV and MRL flow rates to maintain an MRL/MRV ratio, which ratio is determined by one of (a) the difference between the actual and predetermined LNG temperatures and (b) the difference between the actual and predetermined warm end temperature differences or mid-point temperatures; and varying, by an amount corresponding to the other of (b) the difference between the actual and predetermined warm end temperature differences or mid-point temperatures and (a) the difference between the actual and predetermined LNG temperatures, the actual LNG flow rate.