A distance measure is proposed via the gap metric in this paper, and the concept is applied to a boiler-turbine unit to analyze its dynamics. It is shown that the unit shows severe nonlinearity, but the nonlinearity can be avoided by careful choice of the operating range. A single linear controller can be designed to work in such an operating range. It is also shown that the controller constraint is another source of the nonlinearity, which can be compensated using anti-windup techniques. Simulation results are given to verify the conclusions. (C) 2005 Elsevier Ltd. All rights reserved.