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We investigated beam dynamics in a 10-GeV linear accelerator for SASE (Self-Amplified Spontaneous Emission) Free-Electron Laser (FEL) at PAL. The linac is designed to provide the optimal beams to generate radiation of the wavelength of 1-Â¿ in the undulator. An optimum choice of beam parameters is performed to reduce the correlated energy spread after final beam compression and to make accelerator system insensitive to rf jitters. The two-stage bunch compressors are designed to reduce the effects of coherent synchrotron radiation and thus the transverse emittance is not significantly increased. The bunch compressors are also designed such that the effects of nonlinearities due to wakefields, rf curvature and second order momentum compaction become as small as possible in the linear accelerator. With a careful choice of beam and accelerator parameters, the bunch compression process can be made more linear, which minimizes bunch length that can be achieved and helps to reduce high peak current spikes in the beam distributions, which may deteriorate the SASE performance. The tracking simulations in the linear accelerator include longitudinal and transverse wakefields, and the effects of errors such as rf gun timing, rf phase and rf voltage. The simulations also include the X-band rf section and its wakefield. In result, through these design studies, we could get beam parameters and accelerator parameters to be able to provide the radiation power of 6 GW in the wavelength of 1-Â¿ in a 94-m-long undulator. Results on low-charge case of 0.2 nC and the microbunching instability are also shown. It is shown that the designed linac with a tuneable lattice shows satisfactory performances for a 10-GeV X-ray FEL facility.