This work presents design guidelines and performance investigation of plasmonic wavelength demultiplexers for optical communication systems. First the performance of multichannel demultiplexers designed using planar metal-insulator-metal plasmonic structure is investigated. Each demultiplexer unit consists of a bus waveguide supported by two, feedback and drop, nanocavities. Detail parametric study is used to design a high-drop efficiency demultiplexer. The results are then coupled with particle swarm optimization technique to redesign the demultiplexer with high drop efficiency. Simulation is performance using Computer Simulation Technology (CST) package for (1550, 1300, 850 nm) demultiplexer implemented using silver-air- silver plasmonic structure. The design methodology is also used to realize two optical bandpass filters centered around 1650 nm and 1450 nm wavelengths. Then a detail analytic framework to analyze the spectral performance of multichannel demultiplexers designed by cascading single-channel demultiplexing units is presented. The analysis uses coupled-mode theory to deal with incident and reflected waves associated with each demultiplexing unit.