Power optimization is a crucial concern for modern circuit designs. Multiple supply voltages (MSV's) provide an effective technique for the power optimization. This paper addresses the voltage-island generation problem for MSV designs at the post-floorplanning stage. We first present a general formulation of this problem that considers level-shifter planning and power-network routing resources. Without loss of solution quality, we propose an economical graph-based representation that needs only a linear number of nodes to the block number to model the block adjacency in a floorplan for the voltage-island generation. In contrast, previous works need a quadratic number of nodes. To tackle the addressed problem, we employ an ILP formulation which consists of (1) level-shifter aware wirelength estimation to capture the timing overhead, (2) voltage-island-clustering inequalities to avoid complicated constraint transformations, and (3) inequalities to capture the power-network routing-resource usage. Compared with previous works, our algorithm can produce better voltage islands in terms of power-network routing resources. Experimental results show that our algorithm can effectively reduce the power-network routing resource by up to 19.46% with a reasonable overhead of 4.03% more power consumption and using reasonable running time.