In this letter, we focus on a multiuser OFDM system with capability of simultaneous wireless information and power transfer (SWIPT), where information and power are transferred separately on different subcarriers. The optimal design for resource allocation (i.e., power allocation and subcarrier assignment) is investigated to maximize the sum rate with a minimum transferred power constraint. The problem is formulated as a mixed integer programming problem and both optimal and suboptimal solutions as well as the computational complexity are investigated. By comparing the proposed subcarrier-separation (SS) resource allocation scheme with the time-sharing scheme in an existing work as well as an upper bound, we show that the proposed SS scheme outperforms the time-sharing one while high power is required to be transferred.