1. A plant operation support system using a plant simulator, comprising: a parameter modification unit receiving actual data from an actual plant and modifying parameters of a simulation model on the basis of the actual data; a process simulation unit using said simulation model with modified parameters and performing simulations in parallel with operations of said actual plant; and a parameter determination unit using correlations between plant data and variables of said simulation model and selecting parameters to be modified, wherein said parameter modification unit modifies the parameters selected by said parameter determination unit after receiving the actual data from said actual plant, wherein said parameter modification unit modifies the parameters in such a way as to match a simulated output of said process simulation unit with an actual output of the actual plant, wherein the plant operation support system is implemented by a computer, and wherein the simulation model reflects the actual plant.