We consider a joint assortment planning and shelf space allocation problem. The assortment planning sub-problem consists of introducing special products. If these special products are introduced, they can be placed on the shelves that were occupied previously by the standard products. The reduction in the shelf space allocated to the standard products can result in a decrease of the demands for these products. On the other hand, the introduction of the special products can increase the demands for the standard products through cross-selling. The problem is formulated through a nonlinear integer programming problem. The cross-selling effect is estimated by analyzing the real sales data of one year of the biggest supermarket chain of Turkey. We provide numerical results and perform sensitivity analysis to illustrate the effects of cross-selling on our results.