A production planning problem for which the objective function contains both separable and fixed-charge components is formulated under the assumptions of separable constraint functions and a- minimum production level. It is shown that for this problem, the well known application of the λ separation technique can be extended to allow the separation variables, λ, to represent the fixed charge component as well as the nonlinear components of the objective function. The result is the formulation of an approximating problem that has a degree of solution complexity comparable to that of usual separable programs. An application of such formulation to the electric utility generation scheduling problem is presented.