The steric mass action (SMA) model of ion exchange is employed in concert with appropriate mass balance equations to describe the behavior of a concentrated band of protein under gradient-elution conditions. A method of characteristics approach is employed to solve the model equations rapidly. The model is shown to accurately predict the gradient elution of the protein cytochrome C in a strong cation-exchange column. This work constitutes an advance over previously available models as if allows engineers to rapidly consider the effects of nonlinear adsorption in gradient-elution chromatography using a spreadsheet. Further, it provides significant physical insight into the process of gradient elution for preparative and large-scale separations.