[eluser]boltsabre[/eluser]
Not sure what you're doing, but if you are just "removing" some code from a controller to a model (perhaps for the sake of keeping your controller code smaller?), you can technically (and functionally) put it in a model, there is nothing stopping you, it will work. But if this is the case, this code is better moved to either a helper, or externalised into a library, especially if it is code blocks that you are using in more than one controller.
If it's code that that you are only ever going to use in that one controller, you can just isolate it into private function(s)/method(s) at the bottom of your controller class, thus keeping your normal public functions smaller in code size.
It all really depends on what you are doing, and what you want to achieve.