As of the specification Release 13 completed by the 3rd Generation Partnership Project (3GPP) in June 2016, narrowband Internet-of-Things (NB-IoT) has attracted great attention in both academia and industry. Some new features were further specified in subsequent Releases 14 and 15. In light of these specifications, efficient downlink receiver design is critical to the implementation of NB-IoT, due to the strictly limited hardware resources at a receiver. Conforming to Release 15, this paper develops an efficient downlink receiver by jointly accounting for the synchronization, channel estimation and soft combination for repetitive transmissions. Simulation results demonstrate that both the detection probability for the narrowband primary synchronization signal (NPSS) and the block error rate (BLER) for the narrowband physical downlink sharing channel satisfy the benchmarks designated by 3GPP.