I am simulating a cogeneration system, and I am trying to make a load following on the thermal load. Unmet electricity would be made by the grid, and excess would be sold. How can this be possible? As I don't need any boiler, is it possible to remove it?

Unfortunately, HOMER cannot model a system where the thermal load is the primary consideration and the electric load is secondary.  And it is not possible to remove the boiler because HOMER relies on the assumption that the boiler can meet any thermal load that the generators cannot.