“Methodology” implies more than simply the methods you intend to use to collect data. It is often necessary to include a consideration of the concepts and theories which underlie the methods. For instance, if you intend to highlight a specific feature of a sociological theory or test an algorithm for some aspect of information retrieval, or test the validity of a particular system, you have to show that you understand the underlying concepts of the methodology.
There are many different ways to approach the research that fulfills the requirements of a dissertation. These may vary both within and between disciplines. It is important to consider the expectations and possibilities concerning research in your own field. You can do this by talking to your tutors and looking at dissertations written by former students on your course.