The product-mix problem is common and widely applicable in many industries. This paper formulates the product-mix problem for multiple production lines in sequenced stages where the output of the upstream stage is the input for other downstream operations. The objective is to maximize the upstream production (throughput per time) by which throughput of the whole system including the downstream operations will be maximized to satisfy production constraints: productivity, capacity, and sales requirements. The problem is formulated using an integer linear programming (ILP) model and demonstrated in a steel plant in Saudi Arabia. The 480-MT steel plant has six production lines, namely push-pickling line, cold-reversing mill, batch-annealing facility, temper mill, continuous galvanizing line, and color-coating line. The plant intended to leverage its annual production capacity. Hence, a recent expansion was done on the batch annealing facility. After the expansion, the plant had several production interruptions and stoppages in the downstream facilities. The model provided the maximum throughput for all production stages and avoids production interruptions.