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We propose a low-complexity, linear MMSE-based sequential Monte Carlo (SMC) technique as an alternative to the sphere decoder for near-optimal demodulation in MIMO systems. Prior to the SMC procedure, the received signal is passed through a linear MMSE-based preprocessing step, which also determines an optimal channel-dependent order of detection and produces a sequential structure. The algorithm then draws the symbol samples in a deterministic fashion, and the survivor paths are selected based on their importance weights. The proposed algorithm exploits the rectangular structure of the QAM signal constellation by separating the real and imaginary parts of the signal to reduce the complexity associated with the listing and weight update steps. We demonstrate through simulations that the new method achieves the sphere decoder performance for V-BLAST systems.