A process for cooling a tube side stream in a main heat exchanger is described. The process comprises: a) supplying a first mass flow of a tube side stream to a first zone of individual tubes in the tube bundle; b) supplying a second mass flow of the tube side stream to a second zone of individual tubes in the tube bundle, the second zone being offset from the first zone; c) supplying a refrigerant stream on the shell side for cooling the first and second mass flows; d) removing the evaporated refrigerant stream from the warm end of the main heat exchanger; and, e) adjusting the first mass flow of the tube side stream relative to the second mass flow of the tube side stream to maximise the temperature of the removed evaporated refrigerant stream.