How to generalize a polygon file and maintain topology in QGIS?

Question

my problem: I have a polygon-file in shape-format, which I need to generalize. How can "split" the polygons into lines, generalize and than "rebuild" the polygons, so that I can ensure, that there are no topological errors like "slivers". Please give a rookie some adive howto do this in quantumgis.

Thank you very much

Ps:

I already tried:

Vector -> Geometry Tools -> polygones to lines

Vector -> Geometry Tools -> simplfy geometries

here I can already see on the screen, that I am producing overlaps and gaps nevertheless now I try

Vector -> Geometry Tools -> lines to polgones

and here the program show an error and crahes

Answer 1

There is the same discussion here, my final answer being that SAGA does the job for you.

Answer 2

I just did a quick test, and the "Simplify Geometries' tool under the Vector menu appears to respect the topology. I tried with and without the "Enable topological editing" checked off under Snapping Options. Have you tried this?

EDIT: This was done directly on the polygon file--why would you convert to lines first?