This paper deals with the level control of a horizontal closed feedwater heater in a power generation plant. Due to inherent nonlinearities in the system, conventional PI controllers are often tuned conservatively to preserve stability and hence result in suboptimal performance. In this paper, we propose an advanced control strategy that takes into account the system nonlinearities and demonstrate its ability to regulate the heater level at arbitrary setpoints in the presence of various disturbances. Dynamic response tests were carried out on a full-scale simulator of the feedwater heater in a large power plant to verify the performance of the proposed controller. The results show that a performance improvement of at least an order of magnitude can be achieved. It was also observed that the control inputs needed can be implemented on-line using available valve capabilities.