Clearing Functions describe the nonlinear relationship between the orders which are in process and the output in capacitated productions systems under high utilization. These Clearing Function can be derived analytical by using steady-state queuing models or empirical by using simulation. We show how to estimate a Clearing Function for a single-level multi-product production system by using a simulation model. This non-linear Clearing Function is integrated into a model of order release planning We extend this model to consider the uncertainty of the demand by using a Beta service level. We compare our model with an inventory management policy under different demand conditions.