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Optimization for power is always one of the most important design objectives in modern nanometer integrated circuit design. Recent studies have shown the effectiveness of applying multi-bit flip-flops to save the power consumption of the clock network. This paper presents: 1) a novel design methodology of applying multi-bit flip-flops at the post-placement stage, which can be seamlessly integrated in modern design flow; 2) a new problem formulation for post-placement optimization with multi-bit flip-flops; 3) flip-flop clustering and placement algorithms to simultaneously minimize flip-flop power consumption and interconnecting wirelength; and 4) a progressive window-based optimization technique to reduce placement deviation and improve runtime efficiency of our algorithms. Experimental results show that our algorithms are very effective in reducing not only flip-flop power consumption but also clock tree and signal net wirelength. Consequently, the power consumption of the clock network is minimized.