The current invention is related to methods of designing hydrocarbon fluid processing plants and methods of producing hydrocarbon fluids using hydrocarbon fluid processing plants. More particularly, some embodiments of the invention are related to methods of designing natural gas liquefaction plants and methods of producing LNG using natural gas liquefaction plants. One embodiment of the invention includes a method of designing a hydrocarbon fluid processing plant, including: A) providing a process unit configuration for one or more processing units included in a hydrocarbon fluid processing plant; B) determining a cost to capacity relationship for a plurality of equipment types included in the one or more processing units; C) running a process simulation model to obtain a process simulation for the process unit configuration, the process simulation including the estimated capacity of the plurality of equipment types; D) determining a cost measure for the process simulation, the cost measure including an equipment cost measure determined using said estimated capacity of the plurality of equipment types and the cost to capacity relationships; E) altering a process variable in the process simulation model; and F) repeating steps C through E a plurality of times.