An apparatus comprising: at least one processor configured to determine how to adjust one or more manipulated variables to control one or more controlled variables, the one or more manipulated variables and the one or more controlled variables associated with a natural gas liquefaction train (100, 100a-100n) that comprises: a first heat exchanger having a warm end, a cold end, and a mid-point configured to cool a natural gas stream into a liquefied natural gas stream, wherein the first heat exchanger comprises a first path to conduct a methane-rich feed gas that extends from the warm end to the cold end, a second path to conduct a heavy refrigerant that extends from the warm end to the mid-point and a third path to conduct a light refrigerant that extends from the warm end to the cold end; an expansion valve configured to receive the liquefied natural gas stream after the liquefied natural gas stream exits the first heat exchanger and cause the liquefied natural gas stream to expand; and a nitrogen cooling loop comprising a compressor and a second heat exchanger configured to lower a calorific value of the liquefied natural gas stream by further cooling the liquefied natural gas stream after the liquefied natural gas stream exits the expansion valve; wherein the one or more manipulated variables comprise a nitrogen flow to the second heat exchanger of the nitrogen cooling loop in the natural gas liquefaction train, wherein the at least one processor is configured to adjust operation of the compressor associated with the nitrogen cooling loop to adjust the nitrogen flow; and wherein the one or more controlled variables comprise the calorific value of the liquefied natural gas exiting the second heat exchanger of nitrogen cooling loop, wherein the at least one processor is configured to determine how to manipulate the nitrogen flow to control the calorific value of the liquefied natural gas exiting the second heat exchanger of nitrogen cooling loop. at least one processor configured to determine how to adjust one or more manipulated variables to control one or more controlled variables, the one or more manipulated variables and the one or more controlled variables associated with a natural gas liquefaction train (100, 100a-100n) that comprises: a first heat exchanger having a warm end, a cold end, and a mid-point configured to cool a natural gas stream into a liquefied natural gas stream, wherein the first heat exchanger comprises a first path to conduct a methane-rich feed gas that extends from the warm end to the cold end, a second path to conduct a heavy refrigerant that extends from the warm end to the mid-point and a third path to conduct a light refrigerant that extends from the warm end to the cold end; an expansion valve configured to receive the liquefied natural gas stream after the liquefied natural gas stream exits the first heat exchanger and cause the liquefied natural gas stream to expand; and a nitrogen cooling loop comprising a compressor and a second heat exchanger configured to lower a calorific value of the liquefied natural gas stream by further cooling the liquefied natural gas stream after the liquefied natural gas stream exits the expansion valve; a first heat exchanger having a warm end, a cold end, and a mid-point configured to cool a natural gas stream into a liquefied natural gas stream, wherein the first heat exchanger comprises a first path to conduct a methane-rich feed gas that extends from the warm end to the cold end, a second path to conduct a heavy refrigerant that extends from the warm end to the mid-point and a third path to conduct a light refrigerant that extends from the warm end to the cold end; an expansion valve configured to receive the liquefied natural gas stream after the liquefied natural gas stream exits the first heat exchanger and cause the liquefied natural gas stream to expand; and a nitrogen cooling loop comprising a compressor and a second heat exchanger configured to lower a calorific value of the liquefied natural gas stream by further cooling the liquefied natural gas stream after the liquefied natural gas stream exits the expansion valve; wherein the one or more manipulated variables comprise a nitrogen flow to the second heat exchanger of the nitrogen cooling loop in the natural gas liquefaction train, wherein the at least one processor is configured to adjust operation of the compressor associated with the nitrogen cooling loop to adjust the nitrogen flow; and wherein the one or more controlled variables comprise the calorific value of the liquefied natural gas exiting the second heat exchanger of nitrogen cooling loop, wherein the at least one processor is configured to determine how to manipulate the nitrogen flow to control the calorific value of the liquefied natural gas exiting the second heat exchanger of nitrogen cooling loop.