With the help of technology, Philip Seymour Hoffman will still appear in Mockingjay Part 2.

People close to the production are stating that enough of his scenes are finished and fully in tact, meaning they won't replace him with another actor. The current plan is to finish off his final (crucial) scene with stock audio and a virtual replacement for the star.
It didn't work that well with a computer generated Schwarzenegger in Terminator Salvation or Jeff Bridges in Tron Legacy, but with several years worth of advancements, they may be able to pull this off seamlessly.
-CG