NOVELTY - A gaseous, methane-rich feed is liquefied by supplying methane-rich feed at elevated pressure to first tube side (13) of heat exchanger (1), removing evaporated refrigerant from shell side (10) of heat exchanger, compressing in a refrigerant compressor the evaporated refrigerant to get high-pressure refrigerant, and partly condensing the high-pressure refrigerant.    USE - Used in the liquefaction of gaseous, methane-rich feed to obtain liquefied product.    ADVANTAGE - The invention includes control of the bulk composition of the mixed refrigerant.    DETAILED DESCRIPTION - The liquefying of gaseous, methane-rich feed involves supplying methane-rich feed at elevated pressure to first tube side of main heat exchanger at its warm end (3), cooling, liquefying, and sub-cooling the feed against evaporating refrigerant to get liquefied stream, removing the liquefied stream from the heat exchanger at its cold end (5) and passing the liquefied stream to storage as liquefied product; removing evaporated refrigerant from shell side of heat exchanger at its warm end; compressing in the refrigerant compressor the evaporated refrigerant to get high-pressure refrigerant; partly condensing the high-pressure refrigerant and separating in a separator the partly-condensed refrigerant into a liquid heavy refrigerant fraction and a gaseous light refrigerant fraction; sub-cooling the heavy refrigerant fraction in a second tube side (15) of the heat exchanger to get a sub-cooled heavy refrigerant stream, introducing the heavy refrigerant stream at reduced pressure into the shell side of the heat exchanger at its mid-point (7), and allowing the heavy refrigerant stream to evaporate in the shell side; and cooling, liquefying, and sub-cooling part of the light refrigerant fraction in a third tube side (16) of the heat exchanger to get sub-cooled light refrigerant stream, introducing the light refrigerant stream at reduced pressure into the shell side of the heat exchanger at its cold end, and allowing the light refrigerant stream to evaporate in the shell side. The process further comprises adjusting the composition and the amount of refrigerant and controlling the liquefaction process, using an advanced process controller based on a model predictive control to determine simultaneous control actions for a set of manipulated variables in order to optimize a set of parameters while controlling a set of controlled variables. The set of manipulated variables includes mass flow rate of the heavy refrigerant fraction, mass flow rate of light refrigerant fraction, amount of refrigerant components make-up, amount of refrigerant removed, capacity of refrigerant compressor, and mass flow rate of methane-rich feed. The set of controlled variables includes temperature difference at warm end of heat exchanger, variable relating to temperature of liquefied natural gas, composition of refrigerant entering the separator, pressure in the shell of heat exchanger, pressure in the separator, and level of liquid in the separator. The set of variables to be optimized includes production of liquefied product.    DESCRIPTION OF DRAWING(S) - The figure is a flow scheme of a plant for liquefying natural gas.    Heat exchanger (1)    Warm end (3)    Cold end (5)    Mid-point (7)    Shell side (10)    First tube side (13)    Second tube side (15)    Third tube side (16)