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Here are my polygons of town blocks.
The colored towns are considered as areas where they have the same post code.
What I eventually want is to make seamless polygons where there are no gaps between polygons (please see the screenshot below)
What functions should I use to complete my mission?
@Pil Kwon, since you have the same "post-code", therefore in QGIS I can suggest using a "Convex Hull":
Vector > Geoprocessing Tools > Convex Hull
Hint: Create Convex Hulls based on a field.
Wouldn't you want a concave hull? A convex hull will give you a large amount of area that isn't part of the block. For example, a convex hull would fill in that big gap in the "hook" park of block 1.
You can do a morphological closing (buffering outwards and then inwards by the same amount) to close the gaps. It will cause slight distortions to inner corners, though, so your final areas will be changed.
