Here are a few options, depending on the intended meaning:
* from: Duncan is suffering from rheumatism. (This is the most common and natural way to express this)
* with: Duncan is suffering with rheumatism. (This implies the rheumatism is a constant companion or source of pain)
* of: Duncan is suffering of rheumatism. (This is less common but grammatically correct; it emphasizes the feeling of suffering)
The best choice depends on the context and the exact meaning you want to convey.