The design of many core systems-on-chip (SoCs) has become increasingly challenging due to high levels of integration, excessive energy consumption and clock distribution problems. To deal with these issues, we consider network-on-chip (NoC) architectures partitioned into several voltage-frequency islands (VFIs) and propose a design methodology for runtime energy management. The proposed approach minimizes the energy consumption subject to performance constraints. Then, we present efficient techniques for on-the-fly workload monitoring and management to ensure that the system can cope with variability in the workload and various technology-related parameters. Simulation results demonstrate the effectiveness of our approach in reducing the overall system energy consumption for a real video application. Finally, the results and functional correctness are validated using an field-programmable gate-array (FPGA) prototype for an NoC with multiple VFIs.