Integrated large-scale multivariable control and closedloop real-time optimization was implemented on the power plant in Mitsubishi Chemical (Mizushima plant). The multivariable control system controls 3 boilers and 5 turbines simultaneously. Also, the control system controls the site-wide steam header pressure. The realtime optimization system is composed of power plant model and ethylene plant steam turbine model and a number of model's equation is about 10.000. The realtime optimization system optimizes turbine load in the power plant and the site-wide steam balance with about 10 minutes frequency. Most efficient operation of the power plant is realized by the integrated system.