I find that the forshadowing helps to create much of the suspense...just like in Romeo and Juliet, we know from the prologue what is going to happen, but we don't know the how's and when's or other minute details. It keeps us on our toes.
I also love that the book is so different from most of the movie versions out there, so my students are always discussing the differences and how the book was so much better.
I love the fact that several chapters are told from the creature's point of view which gives us a chance to see his side and to feel for him...he is more human than not, and his feelings and emotional responses prove this to be true.
Walton, too, who has identified with Victor throughout the story, has an opportunity to speak with the creature. Up until the very end of the story we don't know what to expect as readers, and neither does our narrator. I am sure he was more than just a little relieved when the creature jumped out the window instead of continuing his rage against Walton and his crew.
I do believe that the suspense in the story is effective. Like most stories of the Romantic era, the author does rely on the "surprise!" technique as much as the blatantly symbolic and emotionally charged explanations of the narrator and the characters. However, Shelley does much to keep the reader on edge.
First of all, the introduction of the outside narrator is particularly clever. Using Walton's letters to his sister as a way of telling the story allows the reader to be an observer. We don't experience with the protagonist; instead, we have the protagonist tell us what is happening. Everything is new and surprising to us.
Secondly, the use of the multiple viewpoints allows us to have just enough information to be scared. When Victor is telling the story, we know enough to be frightened without knowing for sure. Knowing that the monster has been created, but like Victor, not knowing where it is provides suspense. Then, knowing that a death has happened, we can assume that the monster did it - but we don't know for sure. The uncertainty provides fear.
Then, when the monster gets to tell his story, we understand his motivations. While the violent attacks of an unthinking creature were bad enough, the rage of a sentient being is scarier - what will it do? It must have its revenge - what will it be?
Shelley leaves us to wonder, along with Walton, to the very end.