Visible light communication (VLC) is a promising solution for indoor wireless connectivity but its capacity is hindered owing to the light-emitting diodes (LEDs) bandwidth limitation. In this paper, a non-orthogonal multiple access (NOMA) scheme is applied with a multiple-input multiple-output (MIMO) technique for enhancing the achievable rate of VLC systems. As the NOMA performance is extremely affected by the power allocation technique, we evaluate, in an indoor 2x2 MIMO-VLC system based on the NOMA scheme, the performance of three efficient and low-complexity power allocation schemes: fixed power allocation (FPA), gain ratio power allocation (GRPA), normalized gain difference power allocation (NGDPA). According to the results we obtained, the system's best achievable rate was achieved using the FPA with a power allocation factor of 0.9. Moreover, the superiority of FPA over NGDPA and GRPA is clearly highlighted through our simulation results in a two-user case scenario with a sum rate enhancement of 3.7% and 18.7%, respectively.