A quasi-steady-state simulation of contact-angle hysteresis is presented. The model system consists of a two-dimensional drop on a smooth but chemically heterogeneous solid surface. The changes in the free energy of the system, the contact angle, and the size of the base of the drop are followed as its volume increases or decreases. The simulated contact angle hysteresis, including discontinuous changes in the contact angle and the size of the base of the drop, are shown to be very similar to typical experimental observations. In addition, the dependence of receding and advancing angles on the energy available for overcoming energy barriers is illustrated.